Aedes albopictus was introduced into the USA from Asia in the 1980s and now is well established throughout much of the southeast and in several specific localities in the north central U.S. Laboratory studies have demonstrated the ability of this mosquito to transmit LAC and have shown that it can out compete the natural LAC vector, Aedes triseriatus. During the summer and fall of 1997, A. adults, larvae and eggs were collected in Peoria, Illinois, a long recognized focus of LAC encephalitis, and an infected chipmunk was trapped within 150 m of mosquito collection sites. Human encephalitis cases were reported in past years within <2 km of these same sites. These findings suggest a unique opportunity to study the invasion of an urban LAC encephalitis area by A. albopictus, determine the competitive interactions of A. albopictus with the indigenous vector, A. triseriatus, and gather baseline data on the effect of invasion on LAC virus transmission. This pilot study is proposed as an attempt for a rapid research effort on the introduction, spread and establishment of an introduced mosquito and suspected vector into a known focus of enzootic and endemic transmission of LAC encephalitis. The interactions of A. albopictus with A. triseriatus, its feeding preference for mammal reservoirs of LAC virus and its potential as a vector for LAC encephalitis in the field will be studies. If, as hypothesized, A. albopictus becomes established in Peoria, it is likely to displace A. triseriatus may further decline due to intensive control measures against the major nuisance associated with A. albopictus. Mammalian infection rates and ultimately transmission of LAC virus may decline over time (at least in the short term) if vectorial capacity of A. albopictus is lower in the field. The specific aims of this pilot study, to be initiated in the spring 1998, are to determine the persistence and spread of A. albopictus following the mild winter of 1997-98, study the competitive interactions of A. albopictus and A. triseriatus, compare feeding preferences of adult female mosquitoes and determine infection rates of mosquitoes and vertebrate hosts. These data will be linked with landscape/land cover features and meteorological data to determine transmission risk and probable source of human LAC encephalitis cases.